A problem with single overhead rates: Cross subsidizing
If we apply MOH based on Direct Labor
10 orders at 10 units per order will be charged with the same overhead as a single 100 unit order.
Large single set-up orders are cross-subsidizing small multiple set-up orders.

If we try fixing it by using # of batches to allocate MOH then short production runs may absorb too much of the duration driven costs of huge production runs:
A single 4 day production run would absorb the same amount of MOH as a tiny 4 hour run.
The shorter runs are cross-subsidizing the duration driven costs of the long runs.